This invention relates to a drive unit designed particularly for boats fitted with a stern drive, which incorporates a system designed to reverse the propeller rotation.
In particular it relates to a 90-degree transmission which comprises a pair of opposite coaxial bevel gears fitted idle on the engine shaft which engage with a bevel gear integral with the drive shaft, each of which said bevel gears mounted on the engine shaft is fitted with means designed to mesh said shaft with one of said bevel gears in order to control the rotation of the shaft leading to the propellers in one direction or the opposite direction.
Said means are constituted by multi-disk oil-bath clutches, and said bevel gears and said engine shaft are fitted to separate sets of bearings, all of which are mounted on the housing of the device.
Using this solution it is possible to make a stern drive which is very advantageous in terms of compactness, with no need to use a reverse gearbox, which would greatly increase the cost of the unit.
In accordance with the invention, the clutches are housed inside the bevel gears, and the pipes that convey pressurised oil to drive them pass through the shaft to which they are fitted, thus eliminating seal problems.
One of the greatest problems faced by boat designers is how to exploit the little available space to the utmost.
In small and medium-sized boats such as yachts, the problem of exploiting space as effectively as possible relates (among other things) to the size of the engine room, due to the type of drive currently used.
According to the state of the art, systems for the transmission of motion to the propellers can be divided into three categories.
The first and most common type is shown in FIG. 3, where numeral 31 indicates the keel of the boat, only schematized, and 32 indicates the rudder. In this embodiment the engine 33 is connected to the propeller 34 via an inclined shaft 35, with the interposition of a reverse reduction gearbox 36. As the shaft cannot have an inclination greater than 10 degrees, and the size of the propeller cannot be reduced, this configuration obviously has to be very long, with a consequent waste of valuable space, together with lubrication problems.
According to a partial solution to this problem, shown in FIG. 4, the reverse reduction gearbox is the type with conical gears (V-drive type) which allows the engine to be installed horizontally, on the same side of the propeller in relation to the reduction gearbox.
This solution saves a certain amount of space and enables the engine to be ideally positioned, but the reverse/reduction gearbox is very expensive due to the high cost of installing conical gears of sufficient size to transmit the required power.
A further solution, which is the most advantageous in terms of size, is shown in FIG. 5, and involves the use of a stern drive, also known as an “outboard” or strut drive.
In practice, this embodiment relates to a drive unit in which the motion output from the engine is transmitted to a substantially vertical shaft which, via a further transmission, causes the substantially horizontal propeller shaft to rotate.
This known configuration provides the greatest space saving, but it is hardly practical, because a reverse gearbox has to be associated with the transmission device; the overall cost is very high, with the result that this system is uncompetitive, and there is little demand for it.